My grandparents own a dimension 8400 about 2 years old from directly from dell. Recently they started getting an error message stating that the bios battery was low and should be replaced I am just finding it hard to believe the battery is dead already. Could it be something else. Also what should I replace the battery with I have never had to replace a motherboard battery before so I am not sure if it takes a special battery or even where to get such a battery. Thanks for your help
Battery in my 8400 lasted ~2 years so their experience sounds about right. You need a CR2032 (or equivalent) battery. Cost $1.79 at my local electronics store.
Before you replace battery, run BIOS setup (F2 before XP starts to load) and note down all the current settings. Disconnect PC from mains, press/hold power button on tower ~15 sec before opening the case. Remove the battery and insert new one -right side up!
Good idea to use canned air to blow out all the dust bunnies while you're in there too. Don't use a vacuum cleaner because they cause static that can fry the hardware.
When you reboot, run BIOS setup again and make sure all settings are the same as before because removing battery may cause them to be reset to factory defaults.